The present invention relates to methods for generating circuit-breaking signals, in dependence upon the magnitude and the duration of an excess current, where a rectified measured quantity is derived from the excess current and sampled, the samples measured values are converted into corresponding digital values which are then individually weighted in accordance with a predetermined function, and added, and if the sum total exceeds a predetermined comparison value, a circuit-breaking signal is formed, whereas addition is interrupted prior to the formation of the circuit-breaking signal if the excess current falls below a given threshold for a period of time.
A known monitoring device described in German Patent Specification No. 29 50 031 (See U.S. Pat. Nos. 4,219,860 and 4,219,858) operates in accordance with a method of this kind. In this known monitoring device, a measured quantity is obtained from the current which is to be monitored by rectification, sampled by a sampling circuit, and the sampled values obtained in this way are converted into corresponding digital values in an A/D-converter. The digital values are weighted in a function generator, and subsequent to weighting, are added in an adder circuit. When the count of the adder circuit reaches a given comparison value, a circuit-breaking signal is produced. In the known monitoring device, the A/D-converter is connected to a circuit assembly which continuously checks whether the current to be monitored comprises excess current values. When this occurs, a clock-controlled counter connected to the output of the circuit assembly is continuously reset. If the reset signal for the circuit assembly fails to appear, the clock-controlled counter counts up to a predetermined count and then resets the adder circuit. To prevent this from taking place when, on the basis of the curve of the rectified masured quantity, the instantaneous values thereof fall below a value which characterizes an excess current value, in the known monitoring device, the predetermined count must be set to be such that this count can be reached only following a length of time which exceeds the duration of a half-cycle of the current which is to be monitored. In certain situations, this can lead to the circuit being broken even when such is unnecessary. In any case, the known monitoring device which operates in accordance with the described method for currents of 50 Hz or 60 Hz, requires counters which are differently set in respect of the critical count.